1. Field of the Invention
The present invention relates to an optical measurement apparatus including a hemispherical optical integrator for homogenizing light, and particularly to a configuration that enables improvement in integrating efficiency.
2. Description of the Background Art
Recently, development of new light sources such as LED (Light Emitting Diode) and EL (Electro Luminescence) has been advancing rapidly. As an indicator for evaluating such light sources, an indicator such as total luminous flux or color of light of a light source is used. In particular, the total luminous flux of a light source is an important indicator for determining not only the output (lm: lumen) but also the lamp efficiency (lm/W) of the light source.
In order to measure the total luminous flux of such a light source, an integrating sphere (spherical photometer) has been used in which a diffusing material such as barium sulfate is applied to the inner wall of the hollow sphere. In the case where this integrating sphere is used, a light source is lit at the center of the integrating sphere to homogenize the light radiated from the light source and calculate the total luminous flux based on the illuminance of the homogenized light.
Regarding a conventional method of measuring the total luminous flux by means of such an integrating sphere, absorption of light by a jig which is used for securing a light source to be measured to the center of the integrating sphere, and self absorption of the light source itself are error factors. Thus, a method for correcting such errors has been proposed. With such a correction only, however, it has been difficult to precisely measure a light source with which a lighting circuit and a heat-radiation/cooling device are integrated such as high power LED or a surface light source with which an optical system is integrated such as EL or backlight.
As a means for solving such a problem, a hemispherical optical integrator as disclosed in Japanese Patent Laying-Open No. 06-167388 has been proposed.
Further, as an indicator for evaluating a phosphor used for a fluorescent lamp, display or the like, the quantum efficiency is also employed. As a typical means for measuring the quantum efficiency, an optical system for measuring the quantum efficiency of a phosphor is disclosed in “Ohkubo and Shigeta, ‘Absolute Fluorescent Quantum Efficiency of NBS Phosphor Standard Samples,’ Journal of the Illuminating Engineering Institute of Japan, The Illuminating Engineering Institute of Japan, 1999, Vol. 83, No. 2, pp. 87-93” and Japanese Patent Laying-Open No. 10-293063. For such measurement of the quantum efficiency as well, a hemispherical optical integrator as described above can be used.
More specifically, a hemispherical optical integrator as disclosed in Japanese Patent Laying-Open No. 06-167388 includes a hemispherical portion having, on its inner wall, a diffuse reflection layer of a diffusing material such as barium sulfate, and a plane mirror that causes specular reflection (mirror reflection) of light. When the total luminous flux of a light source is to be measured, the light source, which is an object to be measured, is mounted on the plane mirror and at the position of the center of curvature of the hemispherical portion. At this time, a virtual spherical integrating space is created by the hemispherical portion and a virtual image of the hemispherical portion generated by the plane mirror.
A circuit for lighting the light source to be measured and a jig or the like for securing the light source to be measured are located on the opposite side to the hemispherical portion with respect to the plane mirror, and therefore, they can be excluded from this virtual integrating space. Thus, errors due to absorption of light by these circuit and jig or the like can be avoided in principle. In the case where a surface light source is an object to be measured, the light source may be mounted such that only a light emitting portion of the light source is exposed from a window of the plane mirror to thereby avoid the influence of light absorption by a non-light-emitting portion of the object to be measured.
It is noted that U.S. Patent Application Publication US2005/0156103A1 discloses an integrating chamber capable of combining energies of different wavelengths from a plurality of light sources. This integrating chamber merely mixes light rays from the light sources, and does not perform homogenization of the light, which is necessary for measuring the total luminous flux.
As has been described, the hemispherical optical integrator homogenizes light to be measured, by successively reflecting the light from the diffuse reflection layer formed on the inner wall of the hemispherical portion and from the plane mirror. Therefore, it is required to use, for respective reflection surfaces of the inner wall of the hemispherical portion and the plane mirror, a material having a relatively high reflectance for the whole wavelength range of the light to be measured.
However, any material which is to be used for the plane mirror and has a relatively high reflectance particularly in the ultraviolet region or the like of shorter wavelengths is relatively expensive. A problem is therefore that a higher integrating efficiency to be achieved is accompanied by a higher cost of the plane mirror, while a reduced cost to be achieved is accompanied by a lower integrating efficiency.